1. FIELD OF THE INVENTION
This invention relates to a monolithic readout circuit for an uncooled infrared detector and, more specifically, to such a circuit wherein the signal-to-noise ratio of the sensing capacitors is improved.
2. BRIEF DESCRIPTION OF THE PRIOR ART
In order to enhance the performance of infrared detector readout circuits, it is desirable to limit the bandwidth of the detector noise above the signal frequency. For example, with a chop (signal) frequency of 30 Hz, a low pass filter with a 3 db frequency of 60 Hz is desirable. The time constant for the filter would require either a very large resistor or a large capacitor within each unit cell. Using MOS technology, a capacitance of about 1 pF/mil.sup.2 is available, which, for a typical focal plane array (FPA) pitch (1 to 4 mils) yields about 1.0 pF, and combined with typical circuit impedances (1K to 1M ohms), yields a time constant far below that required for a 60 Hz bandwidth. A different approach from that of the prior art is therefore required.
Biasing of a capacitive detector element has been achieved in the prior art by a reset switch in series with the capacitor to be charged which, when closed, provides a low impedance path from the capacitor to a biasing voltage. When the reset switch is then opened, the capacitor retains the charge placed thereon during the period when the reset switch was closed. Voltage setting in this manner leaves a noise voltage behind whose RMS value is equal to (kT/C.sub.det).sup.1/2 where k is Boltzmann's constant and the temperature is measured in degrees Kelvin. This error can be removed by a double correlated sample algorithm in conjunction with a large external memory as described in U.S. Pat. No. 4,671,593. This subtraction requirement is a hindrance and, additionally, is incompatible with the low frequency, low pass filter mentioned above because the reset error pedestal cannot quickly pass through the filter for sampling. Continuous detector biasing through a very high impedance is therefore desirable when a low noise bandwidth is used. Reverse biased diodes have been used for this purpose, however significant leakage current from the detector can create large and non-uniform voltage across the diodes.
High gain for the amplifiers is desirable so that downstream noise, such as that of the filter, is low with respect to detector noise. This is difficult to achieve using typical MOS (not CMOS) common source amplifiers, which require large width to length ratio mismatches (large area) and large bias supplies (increased power). Also, amplifier biasing using a common supply is a problem because of MOS threshold voltage nonuniformity across the array.